The present invention relates to a hydraulic drive system for hydraulic construction machines, such as hydraulic excavators and hydraulic cranes, each equipped with a plurality of hydraulic actuators, and more particularly to a hydraulic drive system for controlling a flow rate of hydraulic fluid supplied to the hydraulic actuators using flow control valves each having a pressure compensating function.
Heretofore, a hydraulic drive system for hydraulic construction machines, such as hydraulic excavators and hydraulic cranes, each equipped with a plurality of hydraulic actuators generally comprises at least one hydraulic pump, a plurality of hydraulic actuators connected to the hydraulic pump through respective main circuits and driven by hydraulic fluid delivered from the hydraulic pump, and a plurality of flow control valves connected to the respective main circuits between the hydraulic pump and the respective hydraulic actuators.
U.S. Pat. No. 4,617,854 discloses a hydraulic drive system of the type having an auxiliary valve disposed in the main circuit upstream of each flow control valve. The inlet and outlet pressures of the flow control valve are both introduced to the first one of opposite operating parts of the auxiliary valve. The delivery pressure of the hydraulic pump and the maximum load pressure among a plurality of hydraulic actuators are both introduced to a second one of the opposite operating parts thereof, and a pump regulator of load sensing type is disposed to hold the delivery pressure of the hydraulic pump at a predetermined value higher than that maximum load pressure. In this arrangement, by introducing the inlet and outlet pressures of the flow control valve to the first one of the opposite operating parts of the auxiliary valve, the load pressure of the flow control valve is compensated as known in the art. Also, by introducing the delivery pressure of the hydraulic pump, regulated by the pump regulator and the maximum load pressure among the plurality of hydraulic actuators, to the second one of the opposite operating parts of the auxiliary valve, in the combined operation of the plurality of hydraulic actuators having respective load pressures different from each other, it is made possible that even if the total of commanded flow rates (required flow rates) of the respective hydraulic actuators exceeds a maximum delivery flow rate of the hydraulic pump, the delivery rate of the hydraulic pump is distributed in accordance with relative ratios of the commanded flow rates to thereby ensure that hydraulic fluid is reliably passed to the hydraulic actuators on the side of higher load pressure as well.
On the other hand, U.S. Pat. No. 4,535,809 discloses a hydraulic drive system directed not to a plurality but to a single hydraulic actuator. In this hydraulic drive system, each flow control valve connected to a main circuit between a hydraulic pump and a hydraulic actuator is constituted by a combination of a main valve of seat valve type, and a pilot valve connected to a pilot circuit between a back pressure chamber of the main valve and an output port. An auxiliary valve is also disposed in the pilot circuit, and the input and output pressures of the pilot valve are introduced to opposite operating parts of the auxiliary valve, respectively, for providing a pressure compensating function. This patent further discloses a modification in which the self-load pressure is used to affect operation of the single hydraulic actuator for correction of the pressure compensating function.
In U.S. Pat. No. 4,617,854, however, the flow control valve and the auxiliary valve each comprise a spool valve which is relatively large in size, as they are both disposed in the main circuit. Accordingly, if the hydraulic circuit is subject to higher pressure for energy savings, this gives rise to the problem of causing appreciable fluid leakage from those spool valves. Also, since the auxiliary valve is disposed in the main circuit through which a large flow rate passes, there is a suffering from another problem of increasing pressure loss at the auxiliary valve.
Generally speaking, each hydraulic actuator in the hydraulic drive system preferably should be supplied with a corresponding flow rate free of any effects from self-load pressure and respective load pressures of other hydraulic actuators. Meanwhile, in some cases, it may be preferable for hydraulic drive systems employed in construction machines such as hydraulic excavators to be affected by load pressures of any other hydraulic actuators or self-load pressure depending on the types of working members and the working modes thereof to be driven by the relevant hydraulic actuator.
For example, when a hydraulic excavator is used for loading earth onto a truck by carrying out swing and boom-up operations concurrently, the load pressure of a swing motor becomes high at the beginning of the swing operation and exceeds the limit pressure of a relief valve provided for circuit protection, because a swing body is of an inertial body. To the contrary, the boom load pressure which represents a boom holding pressure is lower than the swing load pressure. In such working mode, if hydraulic fluid is supplied to the boom to the extent possible rather than being relieved during the time the swing load pressure remains higher at the beginning of the swing operation, energy will be less wasted, and the boom-up and swing operations can automatically be adjusted in their speeds such that the boom-up speed is increased faster than the swing speed at the beginning and, after the boom has been raised up to some extent, the swing speed is gradually increased.
Similarly, in the sole swing operation or the combined swing operation with other hydraulic actuators, the swing load pressure exceeds the limit pressure of a relief valve at the beginning of the swing as mentioned above. Thus, energy will be less wasted provided that the amount of hydraulic fluid supplied to the swing motor can be reduced with the increasing swing load pressure.
In some working modes of a hydraulic excavator, such as normal surface make-up working effected by the combined operation of boom and arm thereof, it is desired to accurately distribute the flow rate in response to the ratio of operated amounts of a boom control lever to an arm control lever irrespective of the magnitude of load pressures.
Therefore, construction machines such as hydraulic excavators desirably have characteristics of the flow control valve which are not determined uniquely for specific pressure compensating and/or flow distributing function, but can be modified to flexibly provide various functions depending on the types of working members and the working modes thereof driven by respective hydraulic actuators.
In U.S. Pat. No. 4,617,854, however, while a pressure compensating function and a flow distributing function can be obtained by provision of the auxiliary valve as mentioned above, there is disclosed no idea of introducing effects from load pressures of other hydraulic actuators or self-load pressure in order to modify those functions. Thus, this patent could not meet the above demand of modifying characteristics of the flow control valve depending on the types of and forms of the working members.
As per U.S. Pat. No. 4,535,809, since it discloses a hydraulic drive system directed to a single hydraulic actuator, provision of the auxiliary valve merely enables it to perform a pressure compensating function in connection with operation of the single hydraulic actuator, or to modify the pressure compensating function by introducing an effect of the self-load pressure of the single hydraulic actuator. Thus, this patent has no relation to the technique of modifying various functions in the combined operation of a plurality of hydraulic actuators. In particular, there is disclosed no idea of introducing effects of load pressures of other hydraulic actuators to modfy the pressure compensating function and the flow distributing function.
It is an object of the present invention to provide a hydraulic drive system which is less subject to a fluid leakage and pressure loss, and which can modify characteristics of a flow control valve depending on the types of working member of hydraulic construction machines and the working modes thereof.